The present invention relates generally to face seal assemblies comprised of a rotating seal rotor and a stationary face seal stator for sealing along the rotating shaft of a gas turbine engine and, in particular, to an improved face seal assembly where at least one of the seal rotor and seal stator is a composite seal rotor and/or a composite seal stator.
Conventional face seal assemblies may be employed in gas turbine engines to prevent leakage of fluid along the engine's rotating shaft where the shaft extends through a wall or partition. These assemblies are comprised of a rotating component called a seal rotor and a non-rotating component called a seal stator. The seal stator is usually lightly spring loaded against the seal rotor.
Historically, various materials have been used for both the seal rotor and seal stator. For example, metals, carbon, ceramics, and other materials are mentioned in Zobens, U.S. Pat. No. 4,174,844; Floyd et al., U.S. Pat. No. 4,036,505; Fenerty et al., U.S. Pat. No. 3,926,443; and Stahl, U.S. Pat. No. 3,770,181. A common configuration is to have a metallic seal rotor and a carbon or graphite stator. A problem with these seals is that oil coking results from the friction between the seal rotor and the seal stator. Also, the carbon or graphite face seal tends to wear which requires that the engine be removed from service regularly to either inspect or replace the seal.
It is well known by those skilled in the art that a carbon or graphite seal stator will wear at a lower rate when rubbing against a ceramic surface as opposed to a metallic surface. Accordingly, one proposal for increasing the life of a conventional face seal assembly is to replace the metallic seal rotor with a ceramic seal rotor, (see for example Fenerty et al., teaching a seal assembly for a water pump in which one of the seal rings is ceramic, column 1, lines 50-55). However, such technology is not applicable to gas turbine engines because the rotating components in these engines are assembled in a lockup. This means that the rotating components (e.g., the compressor disks and turbine disks including the seal rotors) are first stacked one atop the other and then forced, and held together by a large compressive force. This compressive force produces concentrated tensile stresses on the sealing surfaces of the seal rotors abutting a rotating component. Because of its brittle nature conventional ceramic seal rotors tend to crack under these conditions.
To overcome the disadvantages associated with ceramic rotors while maintaining their benefits, Alten, U.S. Pat. No. 5,183,270, discloses a composite seal rotor having an inner metal ring for transmitting compressive forces and an outer ceramic ring for sealingly engaging the carbon face seal. A number of challenges were faced in designing this composite ceramic/metal seal assembly. First, the sealing surfaces must remain extremely flat over a typical operating temperature range of −65 to 400° F. This presents a problem for a composite seal rotor because the various materials combined in the composite have differing thermal expansion behavior which leads to distortion of the assembly during temperature changes. This distortion is sometimes referred to as coning or radial tapering of the sealing face. Second, the mechanism used to hold the metal and ceramic components together must provide sufficient adherence at all temperatures and hold the components together at high rotational speeds, including rotational speeds ranging between 35,000 and 160,000 RPM.
Gasdaska et al., U.S. Pat. No. 6,131,797, discloses a novel brazing method for attaching a metal part to a ceramic part. In particular, this patent discloses a braze joint that includes a layer of bar stock type molybdenum between first and second layers of ductile material. The first ductile layer is brazed to the metal and the second ductile layer is brazed to the ceramic. When the Gasdaska method is used in the formation of a face seal, the seal rotor ends up having seven layers (braze-nickel-braze-molybdenum-braze-nickel-braze) between the metal and ceramic. In many areas of gas turbine engines where face seals are employed, very little space is available making it difficult to use the multi-layer design of the Gasdaska patent.
As can be seen, there is a need for an improved composite face seal assembly that significantly improves performance of the face seal assembly while substantially reducing or eliminating problems with coking that may occur with graphite-to-steel mating surfaces.